1. Field of the Invention
The present invention relates to a contact potential difference ionization detector and method of using such a detector to detect the work function of a surface, and more specifically to a probe for detecting chemical changes on surfaces.
2. Description of Related Art
A Kelvin probe is a name given to a type of sensor that measures the difference in work functions between the probe surface and a surface of interest (the testing surface). This measurement is made by vibrating the probe and detecting an electrical signal related to the vibration frequency. The difference in work function between the Kelvin probe surface and the testing surface results in an electric field when the two surfaces are in electrical contact.
The work function of the surface of an electronic conductor is defined as the minimum amount of work required to move an electron from the interior of the conductor to a point just outside the surface (beyond the image charge region). If an electron is moved through the surface region, its energy is influenced by the optical, electric and magnetic characteristics of the region. Hence, the work function is an extremely sensitive indicator of surface conditions and is affected by absorbed or evaporated layers, surface reconstruction, surface charging, oxide layer imperfections, surface and bulk contamination, and other like properties. Thus, work function measurements are known and can be employed for nondestructive evaluation of surfaces of various materials. See, for example U.S. Pat. No. 5,974,869 which is incorporated by reference herein in its entirety.
The traditional Kelvin probe incorporates a flat circular electrode (termed the reference electrode) suspended above and parallel to a stationary electrode (the specimen), thus creating a capacitor. FIGS. 1a-c illustrate various electron energy diagrams for two different conducting materials. FIG. 1a shows the electron energy level diagram for two conducting specimens that are electrically isolated from one another, where xcfx861 and xcfx862 are the work functions of the materials, and xcex51 and xcex52 represent their Fermi levels. In this case, the Fermi energies and work functions are referenced to a potential in the space between the two materials.
In FIG. 1b it can be seen that if an electrical contact is made between the two electrodes, their Fermi levels equalize by the flow of charge (in the direction indicated). The reference potential is no longer the space between the materials, but is now referenced to the Fermi levels. Relative to the Fermi energy, there develops a potential gradient, termed the contact potential Vc, between the electrodes. The flow of electrons to equilibrate the Fermi levels causes the two surfaces to become equally and oppositely charged.
Referring to FIG. 1c, the inclusion of a variable xe2x80x9cbacking potentialxe2x80x9d Vb in the external circuit permits biasing of one electrode with respect to the other. At the unique point where the (average) electric field between the plates vanishes, there is a null output signal. The work function difference between two surfaces can be found by measuring the flow of charge when the two conducting materials are connected (see FIG. 1b). However, this produces a xe2x80x9conce onlyxe2x80x9d measurement as the surfaces become charged, and the charge must dissipate before another measurement can be made.
By vibrating one of the electrodes (the probe), as has been suggested by Zisman and adapted by many researchers, a varying capacitance is produced, and defined as:
C=Q/V=xcex5rxcex50A/dxe2x80x83xe2x80x83(1)
Where
C is the capacitance;
Q is the Charge;
V is the Potential;
xcex50 is the permittivity of the dielectric (in an air probe the dielectric is air);
xcex5r is the relative dielectric constant;
A is the surface area of the capacitor; and,
d is the separation between the plates.
If the vibration is periodic, then a periodic flow of electrons will result to try to keep the Fermi levels of the two surfaces equal. Therefore, as the separation d increases periodically, the capacitance C decreases periodically.
As the probe oscillates relative to the testing surface, the voltage between the probe and the testing surface can be recorded. If the vibration is sinusoidal, then the peak-to-peak output voltage Vp is given by the equation:
Vp=(xcex94V)RC0xcfx89xcex5sin(xcfx89t+xcfx86)xe2x80x83xe2x80x83(2)
Where
xcex94V represents the voltage between the Kevin probe and the sample;
R is the resistance of the measuring circuit;
C0 is the Kelvin probe capacitance;
xcfx89 is the frequency of vibration;
xcfx86 is the phase angle; and,
xcex5 is the modulation index (d1/d0) where d0 is the average distance between the sample and the probe tip, and d1 is the amplitude of oscillation of the probe.
Yet, there are several disadvantages with capacitance probes like the Kelvin probe as modified by Zisman. One problem with the modified Kelvin probe is that the charge of the electrodes must be dissipated before another measurement can be made, which limits the speed of operation of the probe. Another limitation of the modified or Kelvin type probe arises when used in a gas environment. Measurement of the potential difference in a gas environment by capacitance probes presents problems of reproducibility because adsorption of gas on a surface can cause significant changes in the work function. Such adsorption affects not only the samples being tested, but also the probe. A change in the work function of the probe is virtually indistinguishable from a change in the work function of the sample. Other kinds of surface-gas interactions, as well as changes in environmental conditions such as relative humidity, also can strongly influence the measurements made by Kelvin type probes and other capacitance probes.
Another serious limitation of Kelvin type probes is the need for vibration of the probe. The amplitude of vibration limits how close the probe can be placed relative to the testing surface. The signal will be related to this spacing; the closer the probe can be positioned, the greater the signal and sensitively. Vibration of the probe also is a severe experimental constraints. It necessitates a power source and system to vibrate the probe, and the design of the geometry of the vibrating system. A sensing device that could overcome the many limitations of conventional Kelvin type probes would be beneficial.
Thus, it can be seen that there is a need for the present invention, an improvement over the conventional capacitance probe, by providing a contact potential difference ionization detector that has no moving parts, yet is sensitive enough so as to be capable of sensing gas currents due to the separation of ionized gases by the differences in chemical potential between two different metals. The present invention is primarily directed to the provision of such a non-vibrating probe and its incorporation in a detector, where the signal is related to an ionized gas in the gap of the probe surfaces.
Briefly described, in a preferred form, the present invention provides both a nondestructive testing method of condensed matter surfaces, and a sensing device for the measurement of the work function of the surface of a conducting or semiconducting sample. The present invention can be used to make extremely sensitive motion detectors and accelerometers. This methodology also can be applied to ascertain contact potential for selected non-metals to evaluate changes of chemical state at surfaces.
The present contact potential difference ionization detector comprises an ionization chamber, a probe having a first surface, and a potential difference measurement circuit that is capable of measuring a difference in potential between the first surface of the probe and a testing surface.
The ionization chamber produces ionized particles that travel out of an output of the ionization chamber and into the space between the probe and the testing surface. The probe is non-vibrating having a first surface that is either a positively or negatively charged electrode due to the electrical connection between the first surface and testing surface. The measurement circuit of the present invention is capable of sensing the small amount of electrical current that the ions moving toward the first surface and the testing surface represent.
Thus, an object of the invention is to provide an improved contact potential difference ionization detector.
Another object of the present invention is to provide an improved detector that can be used to make extremely sensitive motion detectors and accelerometers.
Yet another object of the present invention is to provide an improved extremely sensitive method of contact potential ionization detection.
These and other objects, features, and advantages of the present invention will be more apparent upon reading the following specification in conjunction with the accompanying drawings.